When 5e first came out I ran a game in a custom setting. The world was about double the size of Earth, so huge surface area, but not higher gravity due to how much empty space within was occupied by the Underdark. This world had been ruled by all-powerful mage kings until another planet, a cold, dead world from the depths of space, mostly hollow and filled with unnatural beings, collided with it. When this happened it crashed through the upper planes and elemental planes, shards of raw elemental energy fell onto the material plane around it. It landed in the northern hemisphere on top of one of the most powerful magocracies, completely destroying it. During this time of turmoil magic wasn't working right, the boundaries between the upper planes were broken allowing enemies to reach each other, and the gods were occupied battling the powerful elder creatures that had been traveling the void in that hollow world. After an unspecified amount of time the gods built the Shimmering Veil, an inward-facing prismatic sphere that completely surrounded the alien world. The gods have been silent since then, it was believed that their power had been spent. It took a few hundred years for everything to normalize, for magic to become stable again, and for the upper planes to settle down.

When the campaign started, the PCs were refugees on a ship fleeing from the Legion of Souls, an army of slaves commanded by death giants (3.5 MM3) who served a sorceress who fancied herself a goddess. Any who fell in battle with the legion, friend or foe, would have their soul claimed by a nearby giant lieutenant, all of whom paid tribute to the queen. Any who displeased their queen would be executed, their accumulated wealth of souls added to her own. I'd intended for this to be a super-high-level endgame challenge.

The party's ship headed to the far north, to a loose republic of city-states that had been at the fringes of the magocracy that was now under the alien world. It had a cool climate and the light of the Shimmering Veil constantly shone across the land. This region had many areas still infused with raw elemental power, some of which created absurd ecosystems like the jungles around the smoldering crater. The main campaign became them traveling to each of those elemental locations and retrieving an orb of coalesced elemental power, they needed one of each primary element, one of each para-element, and one of each quasi-element. I don't even remember what the purpose of collecting all of them was, probably something to help them fight the death giants, but due to circumstances the game didn't go on long enough for them to find even half the orbs. Granted they probably went on two or three side quests for every orb they found, but that's how it goes sometimes.